Editing Evoked potential (section)

===The "simultaneous stimulation" technique===
This technique allows several (e.g., four) SSEPs to be recorded simultaneously from any given location on the scalp.<ref name="Psychiatry">{{cite journal |author1=Regan D. |author2=Heron J.R. | year = 1969 | title = Clinical investigation of lesions of the visual pathway: a new objective technique | journal = Journal of Neurology, Neurosurgery, and Psychiatry | volume = 32 | issue = 5| pages = 479–83 | doi = 10.1136/jnnp.32.5.479 |pmid=5360055 | pmc = 496563 }}</ref> Different sites of stimulation or different stimuli can be tagged with slightly different frequencies that are virtually identical to the brain, but easily separated by Fourier series analyzers.<ref name="Psychiatry"/> For example, when two unpatterned lights are modulated at slightly different frequencies (F1 and F2) and superimposed, multiple nonlinear cross-modulation components of frequency (mF1 ±  nF2) are created in the SSEP, where m and n are integers.<ref name="electrophysiology"/> These components allow nonlinear processing in the brain to be investigated. By frequency-tagging two superimposed gratings, spatial frequency and orientation tuning properties of the brain mechanisms that process spatial form can be isolated and studied.<ref>{{cite journal |author1=Regan D. |author2=Regan M.P. | year = 1988 | title = Objective evidence for phase–independent spatial frequency analysis in the human visual pathway | doi = 10.1016/S0042-6989(88)80018-X| journal = Vision Research | volume = 28 | issue = 1| pages = 187–191 | pmid = 3413995 |s2cid=21369518 }}</ref><ref>{{cite journal |author1=Regan D. |author2=Regan M.P. | year = 1987 | title = Nonlinearity in human visual responses to two–dimensional patterns and a limitation of Fourier methods | journal = Vision Research | volume = 27 | issue = 12| pages = 2181–3 | doi = 10.1016/0042-6989(87)90132-5 | pmid = 3447366 |s2cid=3175111 }}</ref> Stimuli of different sensory modalities can also be tagged. For example, a visual stimulus was flickered at Fv Hz and a simultaneously presented auditory tone was amplitude modulated at Fa Hz. The existence of a (2Fv + 2Fa) component in the evoked magnetic brain response demonstrated an audio-visual convergence area in the human brain, and the distribution of this response over the head allowed this brain area to be localized.<ref>{{cite journal |author1=Regan M.P. |author2=He P. |author3=Regan D. | year = 1995 | title = An audio–visual convergence area in human brain | journal = Experimental Brain Research | volume = 106 | issue =3 | pages = 485–7 |pmid=8983992 | doi=10.1007/bf00231071|s2cid=27044876 }}</ref> More recently, frequency tagging has been extended from studies of sensory processing to studies of selective attention<ref name="Selective">{{cite journal |author1=Morgan S. T. |author2=Hansen J. C. |author3=Hillyard S. A. | year = 1996 | title = Selective attention to stimulus location modulates the steady-state evoked potential | journal = Proceedings of the National Academy of Sciences USA | volume = 93 | issue = 10| pages = 4770–4774 | doi = 10.1073/pnas.93.10.4770 |pmid=8643478 | pmc= 39354 |doi-access=free }}</ref> and of consciousness.<ref name="Srinivasan">{{cite journal |vauthors=Srinivasan R, Russell DP, Edelman GM, Tononi G | year = 1999 | title = Increased synchronization of neuromagnetic responses during conscious perception | journal = Journal of Neuroscience | volume = 19 | issue = 13| pages = 5435–48 |pmid=10377353| doi = 10.1523/JNEUROSCI.19-13-05435.1999 | pmc = 6782339 | doi-access = free }}</ref>